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Chapter 4
Analog Input
©
National Instruments
4-51
X Series User Manual
Field Wiring Considerations
Environmental noise can serio
u
sly affect the meas
u
rement acc
u
racy of the
Sim
u
ltaneo
u
s MIO X Series device if yo
u
do not take proper care when
r
u
nning signal wires between signal so
u
rces and the device. The following
recommendations apply mainly to AI signal ro
u
ting, altho
u
gh they also
apply to signal ro
u
ting in general.
Minimize noise pick
u
p and maximize meas
u
rement acc
u
racy by taking the
following preca
u
tions:
•
Use individ
u
ally shielded, twisted-pair wires to connect AI signals to
the device. With this type of wire, the signals attached to the AI+ and
AI– inp
u
ts are twisted together and then covered with a shield. Yo
u
then connect this shield only at one point to the signal so
u
rce gro
u
nd.
This kind of connection is req
u
ired for signals traveling thro
u
gh areas
with large magnetic fields or high electromagnetic interference.
•
Ro
u
te signals to the device caref
u
lly. Keep cabling away from noise
so
u
rces. The most common noise so
u
rce in a PCI DAQ system is the
video monitor. Separate the monitor from the analog signals as far as
possible.
•
Separate the signal lines of the Sim
u
ltaneo
u
s MIO X Series device from
high-c
u
rrent or high-voltage lines. These lines can ind
u
ce c
u
rrents in or
voltages on the signal lines of the Sim
u
ltaneo
u
s MIO X Series device if
they r
u
n in close parallel paths. To red
u
ce the magnetic co
u
pling
between lines, separate them by a reasonable distance if they r
u
n in
parallel, or r
u
n the lines at right angles to each other.
•
Do
not
r
u
n signal lines thro
u
gh cond
u
its that also contain power lines.
•
Protect signal lines from magnetic fields ca
u
sed by electric motors,
welding eq
u
ipment, breakers, or transformers by r
u
nning them
thro
u
gh special metal cond
u
its.
Refer to the NI Developer Zone doc
u
ment,
Field Wiring and Noise
Considerations for Analog Signals
, for more information.
Minimizing Drift in Differential Mode
If the readings from the DAQ device are random and drift rapidly, yo
u
sho
u
ld check the gro
u
nd-reference connections. The signal can be
referenced to a level that is considered floating with reference to the device
gro
u
nd reference. Even tho
u
gh yo
u
are in DIFF mode, yo
u
m
u
st still
reference the signal to the same gro
u
nd level as the device reference. There
are vario
u
s methods of achieving this reference while maintaining a high
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